For ventilation hoods in industry to reliably protect workers from airborne contaminants with reasonable energy efficiency, duct systems must distribute the fan airflow appropriately among the branch ducts serving the hoods. However, the distribution of airflows among branch ducts can be skewed by the high levels of "alterations" (e.g., unauthorized modifications, wear, and mechanical damage) suffered by duct systems over their operating lifetimes. As a result, some hoods receive more airflow than necessary while others receive insufficient airflow and provide sharply reduced protection, often with only ambiguous clues to workers and their supervisors that something is amiss. Given the size of typical systems, it is crucial that any "troubleshooting" method both detect the existence of alterations and specify which ducts were altered. Needless searches waste time and labor and encourage fatalistic attitudes about troubleshooting. This field study will determine the efficacy of two current and one proposed troubleshooting method in detecting alterations and in specifying which ducts suffered them. The two current methods associate observed changes in static pressures in a given duct to the presence of an alteration within that duct Since an alteration to any part of the system changes airflows and static pressures everywhere in the system, those methods sometimes identify the wrong ducts. However, laboratory studies have shown that the power loss coefficient determined for a given section of the system is nearly constant unless some duct within that section is altered (e.g.,.by adding an obstruction). Changes to other sections of the duct system or to the fan or air-cleaner do not confuse the picture, as they do with comparisons of static pressures. By comparing before and after coefficient values, one can determine whether one or more alterations were introduced to a particular subsystem, branch, submain, or main, and whether one or more were introduced upstream of any given location. Furthermore, if a given duct is altered, its power loss coefficient will vary monotonically with the effects of the alteration on airflow distribution. In addition, static pressure ratios are proportional to power loss coefficient ratios, so static pressure ratios provide easily determined evidence of alterations. For five industrial exhaust ventilation systems used to protect workers from airborne contaminants, pressures and flows will be measured at six month intervals over a two year period. Ducts with suspected alterations will be identified using each of the procedures and diligently searched for possible alterations. From the number of true and false positives and negatives, it will be possible to compute the sensitivity and specificity for each troubleshooting method. By varying the selection criteria and re- computing sensitivity and specificity, it will be possible to determine and compare the useful diagnostic range of each method. This work will provide challenging field tests for each method. If the proposed method proves superior to existing practices, it could encourage plant personnel to evaluate systems more frequently and more thoroughly, leading to systems that provide higher degrees of protection to workers at lower operating costs.